In recent years, more and more different types of optical fibers have been developed for special applications in optical communication systems, sensing systems, fiber optical gyros, etc. The problem of how to make splices of optical fibers of dissimilar types is therefore often met, in particular in laboratories and factories making gyros and building systems and components utilizing WDM (wavelength division multiplexing) and generally those which use optical building elements obtained from many different manufacturers.
In the following table, characteristic data are listed for some frequently used single mode fibers which have very varying geometric characteristics and mode field diameters (MFD) at their working wavelength.
TABLE 1 ______________________________________ Fiber type Core & No. Producer diameter MFD Wavelength ______________________________________ SM-027 SG/ECA 7.0 mm 10.4 mm 1310 nm SM-004 Optical Fibres 8.0 mm 9.8 mm 1310 nm SM-028 Fujitsu 8.0 mm 9.5 mm 1310 nm SM-016 Corning 7.7 mm 9.4 mm 1310 nm SM-006 AT&T 7.7 mm 9.2 mm 1550 nm DSF-010 Alcatel 5.3 mm 8.0 mm 1550 nm SM-009 Ensign Bickford 5.0 mm 5.7 mm 1060 nm SM-010 Ensign Bickford 4.4 mm 5.0 mm 980 nm EDF-010 Fujitsu 3.5 mm 4.6 mm 1550 nm SM-021 Lycom 3.0 mm 3.6 mm 830 nm EDF-002 Fibercore 3.2 mm 1550 nm ______________________________________
The splice loss of a butt-joint splice due to the MFD mismatch can be approximated by ##EQU1## where, .GAMMA..sub.m is the splice loss due to mismatch of the mode fields, w.sub.1 and w.sub.2 are mode field radii (MFRi) of the two fibers spliced. An illustration of the loss can be found in the diagram of FIG. 1 showing the loss as a function of the ratio w.sub.1 /w.sub.2 of the mode field diameters of two fibers which are spliced to each other. The loss is minimal, i.e. equal to zero, according to Eq. (1) in the case where the two mode field diameters are equal to each other. Of course Eq. (1) gives a theoretical best value whereas in practice the loss will always be greater owing to e.g. lacking basic alignment of fiber cores and deformation of the splicing region.
Considering the fibers listed in Table 1, we find that splices made between two fibers having a large ratio of their MFDs or MFRi will give a splice loss greater than 2 dB, the case being always true for the condition w.sub.1 /w.sub.2 &gt;2. Such a large splice loss will generally result in a large degradation of the quality of the optical system in which the spliced fibers are used. As is well known, the relation between the mode field radius w and the core radius a of an optical fiber can be approximated by, see W. Zheng, "Loss estimation for fusion splices of single-mode fibers", SPIE, Fiber Components and Reliability, Vol. 1580, pp. 380-390, 1991. ##EQU2## wherein V is the normalized frequency: ##EQU3## and n.sub.co and n.sub.cl are refractive indices of the core and the cladding of the fiber respectively and .lambda. is the working wavelength. Since the normalized frequency V should be the same on both sides of a splice point in order to maintain a single mode transmission, we thus have the relation, see K. Shiraishi, T. Yanagi and S. Kawakami, "Light-propagation characteristics in thermally diffused expanded core fiber", IEEE J. of Lightwave Technology, Vol. 11, No. 10, pp.1584-1591, October 1993 EQU a.sub.1.sup.2 .DELTA..sub.1 =a.sub.2.sup.2 .DELTA.2 (5)
The subscripts 1 and 2 in Eq. 5 represent the left and right fibers of the splice. One can clearly observe from the relation that for a fiber having a smaller core radius, the difference between the refractive indices of the core and the cladding should be higher. During the heating when the fiber ends are fusioned to each other, the higher difference of the refractive indices will result in a higher diffusion speed of the core material as discussed in W. Zheng, O. Hulten and R. Rylander, "Erbium-doped fiber splicing and splice loss estimation." IEEE J. of Lightwave Technology., Vol. 12, No. 3, pp. 430-435, 1994. Such different diffusion speeds tend to enlarge the smaller MFR faster than the larger MFR. When continuing the heating process, one can always find a moment, when the lowest splice loss due to mismatch of the MFRi can be obtained. There are several methods of making this loss optimization as discussed in the cited paper by W. Zheng, O. Hulten et al. One of the most useful methods using digital image processing is the hot-fiber index monitoring technique as described in the same paper and Swedish patent applications Nos. 9201818-3 and 9201817-5, which are all incorporated herein by reference.